The present invention relates to a method for densifying or compacting fillers and to the compositions made by such method. More particularly, the present invention relates to a method for densifying reinforcing fillers such as fumed silica and precipitated silica or extending fillers such as ground quartz and calcium carbonate by blending such filler with a silicone polymer to provide free-flowing powders having higher densities than can be obtained using present densification methods.
Reinforcing fillers such as fumed silica and precipitated silica are well known in the prior art. Basically, these fillers are silicon dioxide particles in extremely finely divided form, that is, they have a particle size of less than 1 micron, preferably less than 500 millimicrons and most preferably less than 100 millimicrons. The specific surface area of the substances therefore correspondingly is between several square meters per gram and several hundred square meters per gram, for example, between 10 and 400 square meters per gram. Similarly, the density correspondingly ranges from about 3 to about 5 pounds per cubic foot. Generally such reinforcing fillers are added to a silicone composition in order to provide high tensile strength and hardness.
One method for the production of extremely finely divided silica is the pyrogenic hydrolysis of volatile silicon halides, preferably chlorosilanes, to produce oxides in the gas phase according to the reaction equation (for silicone tetrachloride): EQU SiCl.sub.4 +2H.sub.2 +O.sub.2 .fwdarw.SiO.sub.2 +4HCl
Such methods of pyrogenic hydrolysis are well known in the art, for example, U.S. Pat. No. 3,954,945, U.K. Pat. No. 1,121,278 and U.K. Pat. No. 1,121,279.
Another known method for the production of extremely finely divided silicon dioxide particles is the precipitation of silica from alkali metal silicate solutions with acids such as, for example, carbonic acid or sulfuric acid. The gel thus obtained is filtered off, washed and dried and, if necessary, ground so as to provide a precipitated silica filler having a surface area between about ten and several hundred square meters per gram.
It is furthermore known that mixtures of oxidic substances and carbon can be given a reducing treatment in an electric arc furnace in such a manner that a gas mixture of carbon monoxide and metal oxide is obtained and which is then burned, taking care that the combustion is complete and that the temperature is sufficiently low that agglomeration is avoided. Such a process is described, for example, in U.S. Pat. No. 2,862,792 and U.S. Pat. No. 3,311,451.
Those skilled in the art are familiar with the foregoing methods of manufacturing reinforcing fillers as well as other suitable methods.
Extending fillers, for example, ground quartz, diatomaceous earth, zinc oxide, titanium dioxide, calcium carbonate, iron oxides and the like are also well known in the art. Extending fillers are typically included in a composition not only to lower costs, but also to increase durometer and to reduce elongation.
While reinforcing fillers are usually critical in formulating silicone compositions suitable for a particular purpose, such fillers suffer from the disadvantage that due to their low density they are rather expensive to ship and store. A compounder or formulator who requires large quantities of fillers must pay premium rates for shipping, for example, by rail car, because a rail car cannot contain large amounts, on a weight basis, of such fillers. Moreover, once the compounder receives the fillers he must pay for a suitable storage area such as a warehouse, silo or the like, which further adds to the costs which are ultimately passed on to the final customers or end-users.
Accordingly it would be highly desirable to provide a means for increasing the density of fillers in order to reduce transportation and shipping costs. Furthermore, it would also be highly desirable to provide a densified reinforcing or extending filler which is easily and quickly mixed into other silicone polymers so as to increase efficiency and productivity.
An apparatus for densifying and granulating powdered materials is disclosed in Oldham et al., U.S. Pat. No. 3,114,930. Oldham et al. relies on the use of a vacuum to remove air from an aerated powdered material thereby reducing the density.
Loffler in U.S. Pat. No. 3,632,247 expands upon the invention of Oldham et al. in that powders are compressed and deaerated between vacuum cylinders which are arranged in groups requiring different vacuum and connected to a common vacuum line. Valve control means in the vacuum line automatically and continuously adjust the vacuum pressure for the groups of cylinders.
Carter, U.S. Pat. No. 3,664,385, compacts finely divided particulate material by utilizing a rotating screw feeder. The particulate material is advanced axially along a sleeve with the interstitial air between the particles in the sleeve at an internal sleeve pressure. Suction pressure relatively lower than the internal sleeve pressure is applied to the exterior of the sleeve to withdraw air from between the particles of the material to effect compaction.
U.S. Pat. No. 4,126,423 to Kongsgaarden discloses a method for compacting silica dust wherein the dust is charged to a drum having closed ends and is tumbled therein. Kongsgaarden, in U.S. Pat. No. 4,126,424, provides an alternate method for compacting silica dust which comprises charging the dust to a hopper and thereafter injecting pressurized air into the hopper. The air is of sufficient force to fluidize the silica dust in the hopper and maintain it in an agitated state. When the treatment with air has been completed, the bulk density of the material has increased by up to 300%.
Leon et al., in U.S. Pat. No. 4,325,686, disclosed a powder densifying apparatus comprising a pair of opposed gas-permeable belts arranged to either side of a common axis so as to define a generally convergent densifying zone between their adjacent faces. The gas-permeable belts are driven toward the convergent end of the densifying zone at substantially equal speeds while powder material to be densified is fed into the divergent end of the densifying zone at a rate sufficient to maintain a substantially complete fill thereof.
Kratel et al., U.S. Pat. No. 4,326,852, provides a method for increasing the bulk weight of silicon dioxide with a surface of at least 50 square meters per gram by means of sub-atmospheric pressure applied at a filter face, wherein the silicon dioxide is moved by means of a conveyor screw, whose longitudinal axis is arranged parallel with respect to the filter face and which preferably has a decreasing thread pitch in the feeding direction.
While fillers prepared in accordance with the foregoing patents are substantially more dense than without including a densification step, it is nevertheless difficult, time consuming and rather costly to incorporate such fillers into a viscous, tacky silicone polymer. Moreover, the density is still not such that substantial savings in shipping and storage can be attained.
Accordingly, it is highly desirable to provide a means for providing even more dense reinforcing and extending fillers. Furthermore, it is desirable to provide such dense fillers which are also easily and quickly mixed with other silicone polymer.